4-7-8 Breath
Utterly simple, takes almost no time, requires no equipment, can be done anywhere

Although you can do the exercise in any position, sit with your back straight while learning the exercise. Place the tip of your tongue against the ridge of tissue just behind your upper front teeth, and keep it there through the entire exercise. You will be exhaling through your mouth around your tongue; try pursing your lips slightly if this seems awkward.

Exhale completely through your mouth, making a whoosh sound.
Close your mouth and inhale quietly through your nose to a mental count of four.
Hold your breath for a count of seven.
Exhale completely through your mouth, making a whoosh sound to a count of eight.
This is one breath. Now inhale again and repeat the cycle three more times for a total of four breaths.